Signal analysis apparatus and signal analysis method

ABSTRACT

A signal analysis apparatus includes a first frequency conversion unit 10 that generates an intermediate frequency signal SIF2 and a second spurious signal SSP2 from a measured signal SRF of a frequency fRF (center frequency fc), and a second frequency conversion unit 25 that converts the intermediate frequency signal SIF2 and the second spurious signal SSP2 into an intermediate frequency signal SIF2′ of a frequency fIF2′ and a second spurious signal SSP2′ of a frequency fSP2′ by performing frequency shift of the intermediate frequency signal SIF2 and the second spurious signal SSP2 by a frequency shift amount Δf, in which the frequency shift amount Δf is a value that does not establish a relationship of −W/2≤fSP2′−fIF2′≤+W/2.

TECHNICAL FIELD

The present invention relates to a signal analysis apparatus and a signal analysis method and particularly, to a signal analysis apparatus and a signal analysis method using a superheterodyne method.

BACKGROUND ART

In the related art, a signal analysis apparatus that receives an RF signal of a wide band output from a device under test as a measured signal and performs frequency analysis on the measured signal has been known (for example, refer to Patent Document 1).

For example, as illustrated in FIG. 5, the signal analysis apparatus of the related art includes a first mixer 61, a first bandpass filter (BPF) 62, a second mixer 63, a second BPF 64, an AD converter (ADC) 65, and a signal analysis unit 66.

The first mixer 61, by performing frequency mixing of a measured signal of a frequency f_(RF) (center frequency f_(c)) with a first local oscillation signal of a frequency f_(LO1), generates output signals of a sum component and a difference component of frequencies of the two signals, that is, output signals of frequencies f_(RF)+f_(LO1) and |f_(RF)−f_(LO1)|. The first BPF 62 allows the output signal of the difference component from the first mixer 61 to pass as an intermediate frequency signal having an intermediate frequency f_(IF) and removes the output signal of the sum component from the first mixer 61.

The second mixer 63, by performing the frequency mixing of the signal passing through the first BPF 62 with a second local oscillation signal of a frequency f_(LO2), generates output signals of a sum component and a difference component of the frequencies of the two signals. The second BPF 64 allows the output signal of the difference component from the second mixer 63 to pass and removes the output signal of the sum component from the second mixer 63.

The ADC 65 performs AD conversion of the signal passing through the second BPF 64 into binary digital data having a predetermined number of bits by sampling the signal at a predetermined sampling frequency. The signal analysis unit 66 analyzes the signal after the AD conversion performed by the ADC 65.

A harmonic of the measured signal is generated in the first mixer 61, and the harmonic is input into the first BPF 62. For example, a case where a passed center frequency of the first BPF 62 is 9.8 GHz, the center frequency f_(c) of the measured signal is 4.9 GHz, and the frequency f_(LO1) of the first local oscillation signal is 14.7 GHz is considered. Here, a problem arises in that a second harmonic of the measured signal appears as a spurious signal within a frequency range centered at 9.8 GHz and cannot be distinguished due to overlap with the frequency f_(RF) of a desired wave of the measured signal.

Furthermore, as illustrated in FIG. 6, a frequency component of 4.85 GHz in the measured signal having the center frequency f_(c) of 4.9 GHz is handled as a spurious signal of 9.7 GHz by the first mixer 61. Meanwhile, a center frequency of the desired wave obtained as the output signal of the difference component from the first mixer 61 is 9.85 GHz. Here, in a case where frequency components corresponding to 9.7 GHz of the spurious signal and 9.85 GHz of the desired wave of the measured signal are included in a passed bandwidth W of the second BPF 64 of a rear stage, there is a possibility of an adverse effect on a signal analysis result of the signal analysis unit 66.

RELATED ART DOCUMENT Patent Document

[Patent Document 1] Japanese Patent No. 5947943

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

In order to avoid the above problem of being unable to distinguish the harmonic of the measured signal generated by the first mixer 61 due to overlap with the desired wave of the measured signal, a frequency relationship in which the harmonic and the desired wave of the measured signal do not overlap may be selected upon designing. For doing so, for example, a design that increases the passed center frequency of the first BPF 62 is considered. However, a problem arises in that a design difficulty or a cost of hardware related to frequency conversion including the first BPF 62 rises.

The present invention is conceived in order to solve such a problem of the related art, and an object thereof is to provide a signal analysis apparatus and a signal analysis method capable of analyzing a measured signal while avoiding a spurious harmonic of the measured signal generated by a mixer, without changing a configuration of hardware related to frequency conversion.

Means for Solving the Problem

In order to solve the above problem, a signal analysis apparatus according to the present invention is a signal analysis apparatus that analyzes a measured signal having a frequency component f_(RF) and includes a frequency setting unit that decides a first intermediate frequency f_(IF1), a frequency f_(LO1) of a first local oscillation signal, and a frequency shift amount Δf in accordance with a center frequency f_(c) of the measured signal, a first local oscillator that outputs the first local oscillation signal, a first mixer that generates a first intermediate frequency signal of the first intermediate frequency f_(IF1) by performing frequency mixing of the measured signal with the first local oscillation signal and generates a first spurious signal including a second harmonic component of the measured signal, a bandpass filter of a passed center frequency f₀ into which the first intermediate frequency signal and the first spurious signal are input, a second local oscillator that outputs a second local oscillation signal of a frequency f_(LO2), a second mixer that generates a second intermediate frequency signal of a second intermediate frequency f_(IF2) by performing frequency mixing of the first intermediate frequency signal passing through the bandpass filter with the second local oscillation signal and generates a second spurious signal by performing frequency mixing of the first spurious signal passing through the bandpass filter with the second local oscillation signal, an AD converter that performs AD conversion of the second intermediate frequency signal and the second spurious signal, a frequency shift unit that performs frequency shift of the second intermediate frequency signal and the second spurious signal after the AD conversion performed by the AD converter by the frequency shift amount Af on a frequency axis, a digital filter of a passed bandwidth W that allows the second intermediate frequency signal after the frequency shift performed by the frequency shift unit to pass and removes the second spurious signal after the frequency shift performed by the frequency shift unit, and a signal analysis unit that analyzes the second intermediate frequency signal passing through the digital filter, in which the first intermediate frequency f_(IF1) is provided as f₀+Δf, the second intermediate frequency f_(IF2) is provided as |f₀+Δf−f_(LO2)|, a second intermediate frequency f_(IF2)′ of the second intermediate frequency signal after the frequency shift performed by the frequency shift unit is provided as |f₀−f_(LO2)|, the frequency f_(Lo1) is provided as f_(c)+f₀+Δf, a frequency f_(SP1) of the first spurious signal is provided as 2×f_(RF), a frequency f_(SP2) of the second spurious signal is provided as |f_(SP1)−f_(LO2)|, a frequency f_(SP2)′ of the second spurious signal after the frequency shift performed by the frequency shift unit is provided as |f_(SP1)−f_(LO2)−Δf|, and the frequency setting unit decides the frequency shift amount Δf that does not establish a relationship of −W/2≤|f_(SP1)−f_(LO2)−Δf|−f_(IF2)′≤+W/2, and decides the first intermediate frequency f_(IF1) and the frequency f_(LO1).

According to this configuration, the signal analysis apparatus according to the present invention decides the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1) in accordance with the center frequency f_(c) of the measured signal, performs an analog signal process, and then, performs a digital signal process. Accordingly, by varying the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1), the signal analysis apparatus according to the present invention can analyze the measured signal while avoiding a spurious harmonic of the measured signal generated by the first mixer, without changing a configuration of hardware related to frequency conversion. Furthermore, even in a case where a standard that uses a new band is established in the future, the signal analysis apparatus according to the present invention can perform measurement that avoids a spurious harmonic, without changing the configuration of the hardware related to the frequency conversion.

In addition, in the signal analysis apparatus according to the present invention, in a case where a relationship of −W/2≤|f_(SP1)−f_(LO2)|−f_(IF2)′≤+W/2 is not established, the frequency setting unit may decide the frequency shift amount Δf as Δf=0.

In addition, in the signal analysis apparatus according to the present invention, in a case where a relationship of −W/2≤|f_(SP1)−f_(LO2)|−f_(IF2)′≤+W/2 is established, the frequency setting unit may decide the frequency shift amount Δf capable of allowing the second intermediate frequency signal to pass and removing the second spurious signal at the digital filter.

According to these configurations, since the first intermediate frequency f_(IF1) and the frequency f_(LO1) can be separately used by shifting the first intermediate frequency f_(IF1) and the frequency f_(LO1) by the frequency shift amount Δf, the signal analysis apparatus according to the present invention can receive a desired wave of the second intermediate frequency f_(IF2)′ while removing an unnecessary spurious harmonic of the frequency f_(SP2)′ using the digital filter of the passed bandwidth W. In other words, the signal analysis apparatus according to the present invention can separate the first intermediate frequency f_(IF1) and the frequency f_(SP1) of the first spurious signal generated by the first mixer, using the digital filter of a rear stage.

In addition, a signal analysis method according to the present invention is a signal analysis method for analyzing a measured signal of a frequency f_(RF) and includes a frequency setting step of deciding a first intermediate frequency f_(IF1), a frequency f_(LO1) of a first local oscillation signal, and a frequency shift amount Δf in accordance with a center frequency f_(c) of the frequency f_(RF), a first local oscillation step of outputting the first local oscillation signal of the frequency f_(LO1), a first frequency mixing step of generating a first intermediate frequency signal of the first intermediate frequency f_(IF1) by performing frequency mixing of the measured signal with the first local oscillation signal, and generating a first spurious signal including a second harmonic component of the measured signal, a bandpass step of a passed center frequency f₀ into which the first intermediate frequency signal and the first spurious signal are input, a second local oscillation step of outputting a second local oscillation signal of a frequency f_(LO2), a second frequency mixing step of generating a second intermediate frequency signal of a second intermediate frequency f_(IF2) by performing frequency mixing of the first intermediate frequency signal passing through the bandpass step with the second local oscillation signal, and generating a second spurious signal by performing frequency mixing of the first spurious signal passing through the bandpass step with the second local oscillation signal, an AD conversion step of performing AD conversion of the second intermediate frequency signal and the second spurious signal, an NCOM step of performing frequency shift of the second intermediate frequency signal and the second spurious signal after the AD conversion performed in the AD conversion step by the frequency shift amount Δf on a frequency axis, a digital filter step of a passed bandwidth W in which the second intermediate frequency signal after the frequency shift performed in the NCOM step is allowed to pass, and the second spurious signal after the frequency shift performed in the NCOM step is removed, and a signal analysis step of analyzing the second intermediate frequency signal passing through the digital filter step, in which the first intermediate frequency f_(IF1) is provided as f₀+Δf, the second intermediate frequency f_(IF2) is provided as |f₀+Δf−f_(LO2)|, a second intermediate frequency f_(IF2)′ of the second intermediate frequency signal after the frequency shift performed in the NCOM step is provided as |f₀−f_(LO2)|, the frequency f_(LO1) is provided as f_(c)+f₀+Δf, a frequency f_(SP1) of the first spurious signal is provided as 2×f_(RF), a frequency f_(SP2) of the second spurious signal is provided as |f_(SP1)−f_(LO2)|, a frequency f_(SP2)′ of the second spurious signal after the frequency shift performed in the NCOM step is provided as |f_(SP1)−f_(LO2)−Δf|, and in the frequency setting step, the first intermediate frequency f_(IF1) and the frequency f_(LO1) are decided using the frequency shift amount Δf that does not establish a relationship of −W/2≤|f_(SP1)−f_(LO2)−Δf|−f_(IF2)′≤+W/2.

According to this configuration, in the signal analysis method according to the present invention, the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1) are decided in accordance with the center frequency f_(c) of the measured signal, an analog signal process is performed, and then, a digital signal process is performed. Accordingly, by varying the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1), in the signal analysis method according to the present invention, the measured signal can be analyzed while avoiding a spurious harmonic of the measured signal generated by the first mixer, without changing a configuration of hardware related to frequency conversion. Furthermore, even in a case where a standard that uses a new band is established in the future, in the signal analysis method according to the present invention, measurement that avoids a spurious harmonic can be performed without changing the configuration of the hardware related to the frequency conversion.

Advantage of the Invention

The present invention provides a signal analysis apparatus and a signal analysis method capable of analyzing a measured signal while avoiding a spurious harmonic of the measured signal generated by a mixer, without changing a configuration of hardware related to frequency conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a signal analysis apparatus according to an embodiment of the present invention.

FIG. 2 is a table illustrating a frequency relationship of each unit constituting a frequency conversion unit of the signal analysis apparatus according to the embodiment of the present invention.

FIGS. 3A and 3B are diagrams schematically illustrating a frequency relationship among a bandwidth of a measured signal immediately after being output from a first BPF, a bandwidth of a first spurious signal, and a passed bandwidth of a second BPF: FIG. 3A illustrates a case where a center frequency f_(c) of the measured signal is within a range of 4.45 GHz≤f_(c)≤4.5 GHz, and FIG. 3B illustrates a case where the center frequency f_(c) of the measured signal is within a range of 4.5 GHz<f_(c)≤4.55 GHz.

FIG. 4 is a flowchart for describing a process of a signal analysis method using the signal analysis apparatus according to the embodiment of the present invention.

FIG. 5 is a block diagram illustrating a configuration of a signal analysis apparatus of the related art.

FIG. 6 is a diagram schematically illustrating a relationship between a desired wave and a spurious signal in the signal analysis apparatus in FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of a signal analysis apparatus and a signal analysis method according to the present invention will be described using the drawings.

As illustrated in FIG. 1, a signal analysis apparatus 1 according to the embodiment of the present invention is an apparatus for analyzing a measured signal S_(RF) of a frequency f_(RF) (center frequency f_(c)) output from a device under test (DUT) 100 and includes a first frequency conversion unit 10, an AD converter 20, a second frequency conversion unit 25, a frequency setting unit 30, a signal analysis unit 40, a display unit 50, an operation unit 51, and a control unit 52.

For example, the DUT 100 is a mobile terminal or a base station that includes a wireless communication antenna and an RF circuit and can output an analog RF signal. Examples of a communication standard of the DUT 100 include 5G NR, TD-LTE, FDD-LTE, LTE-Advanced, GSM (registered trademark), TD-SCDMA, W-CDMA (registered trademark), CDMA2000, and Bluetooth (registered trademark). The DUT 100 and the first frequency conversion unit 10 may be connected in a wired manner through a coaxial cable or the like, or may be connected in a wireless manner through a wireless communication antenna.

The first frequency conversion unit 10 performs frequency conversion of a frequency of the analog RF signal as the measured signal S_(RF) output from the DUT 100 and includes a first local oscillator 11, a first mixer 12, a BPF 13, a second local oscillator 14, and a second mixer 15.

For example, the first local oscillator 11 is configured with a PLL circuit and receives a control signal from the frequency setting unit 30, described later, and outputs a sine wave of a frequency f_(LO1) that is higher than the original frequency f_(RF) of the measured signal S_(RF) by a frequency of a conversion result, to the first mixer 12 as a first local oscillation signal S_(LO1).

The first mixer 12, by performing frequency mixing of the measured signal S_(RF) of the frequency f_(RF) with the first local oscillation signal S_(LO1) of the frequency f_(LO1) output from the first local oscillator 11, generates output signals of a sum component and a difference component of the frequencies of the two signals, that is, output signals of frequencies f_(RF)+f_(LO1) and |f_(RF)−f_(LO1)|. In addition, the first mixer 12 generates an output signal including a second harmonic component of the measured signal S_(RF). Hereinafter, the frequency |f_(RF)−f_(LO1)| will be referred to as a “first intermediate frequency f_(IF1)”, and the output signal of the frequency |f_(RF)−f_(LO1)| will be referred to as a “first intermediate frequency signal S_(IF1)”. In addition, the signal including the second harmonic component of the measured signal S_(RF) will be referred to as a “first spurious signal S_(SP1)”.

The BPF 13 is a bandpass filter of a passed center frequency f₀ disposed between the first mixer 12 and the second mixer 15, and output signals including the first intermediate frequency signal S_(IF1) and the first spurious signal S_(SP1) output from the first mixer 12 are input into the BPF 13. The BPF 13 allows the first intermediate frequency signal S_(IF1) of the first intermediate frequency f_(IF1) and the first spurious signal S_(SP1) to pass among the output signals from the first mixer 12, and removes the sum component, a third or higher harmonic component of the measured signal S_(RF), and a harmonic component of the first local oscillation signal S_(LO1) among the output signals from the first mixer 12.

For example, the second local oscillator 14 is configured with a PLL circuit and outputs a sine wave of a frequency f_(LO2) to the second mixer 15 as a second local oscillation signal S_(LO2). Here, the frequency f_(LO2) is a fixed value.

The second mixer 15, by performing the frequency mixing of the first intermediate frequency signal S_(IF1) of the first intermediate frequency f_(IF1) passing through the BPF 13 with the second local oscillation signal S_(LO2) of the frequency f_(LO2) output from the second local oscillator 14, generates output signals of a sum component and a difference component of the frequencies of the two signals, that is, output signals of frequencies f_(IF1)+f_(LO2) and |f_(IF1)−f_(LO2)|. In addition, the second mixer 15, by performing the frequency mixing of the first spurious signal S_(SP1) of the frequency f_(SP1) passing through the BPF 13 with the second local oscillation signal S_(LO2) of the frequency f_(LO2) output from the second local oscillator 14, generates output signals of a sum component and a difference component of the frequencies of the two signals, that is, output signals of frequencies f_(SP1)+f_(LO2) and |f_(SP1)−f_(LO2)|.

Hereinafter, the frequency |f_(IF1)−f_(LO2)| will be referred to as a “second intermediate frequency f_(IF2)”, and the output signal of the frequency |f_(IF1)−f_(LO2)| will be referred to as a “second intermediate frequency signal S_(IF2)”. In addition, the frequency |f_(SP1)−f_(LO2)| will be referred to as a “frequency f_(SP2)”, and an output signal of the frequency f_(SP2) will be referred to as a “second spurious signal _(Ssp2”.) That is, the second spurious signal S_(SP2) is an unnecessary wave signal downconverted from the first spurious signal S_(SP1) by the second mixer 15.

The AD converter 20 performs AD conversion of output signals including the second intermediate frequency signal S_(IF2) and the second spurious signal S_(SP2) output from the second mixer 15 into binary digital data having a predetermined number of bits by sampling the output signals at a predetermined sampling frequency.

The second frequency conversion unit 25 performs frequency conversion of the output signals from the AD converter 20 and includes an NCOM unit (frequency shift) 26 and a digital filter 27.

For example, the NCOM unit 26 includes a numeric controlled oscillator modulator (NCOM), a digital up converter (DUC), and a digital down converter (DDC) and performs frequency shift of the output signals including the second intermediate frequency signal S_(IF2) and the second spurious signal S_(SP2) after the AD conversion performed by the AD converter 20 by a frequency shift amount Δf on a frequency axis.

For example, the digital filter 27 is a bandpass filter of a passed center frequency f_(D) and a passed bandwidth W and allows a second intermediate frequency signal S_(IF2)′ after the frequency shift performed by the NCOM unit 26 to pass and removes a second spurious signal S_(SP2)′ after the frequency shift performed by the NCOM unit 26. Here, the passed center frequency f_(D) is a fixed value equal to f_(IF2)′ described later.

The first intermediate frequency f_(IF1), the second intermediate frequency f_(IF2), the second intermediate frequency f_(IF2)′ after the frequency shift performed by the NCOM unit 26, the frequency f_(LO1) of the first local oscillation signal S_(LO1), the frequency f_(SP1) of the first spurious signal S_(SP1), the frequency f_(SP2) of the second spurious signal S_(SP2), and a frequency f_(SP2)′ of the second spurious signal S_(SP2)′ after the frequency shift performed by the NCOM unit 26 are provided by Expressions (1) to (7) below.

f _(IF1) =f ₀ +Δf  (1)

f _(IF2) =|f _(IF1) −f _(LO2) |=f ₀ +Δf−f _(LO2)|  (2)

f _(IF2) ′=f _(D) =|f ₀ −f _(LO2)|  (3)

f _(LO1) =f _(c) +f _(IF1) =f _(c) +f ₀ +Δf  (4)

f _(SP1)=2×f _(RF)  (5)

f _(SP2) =|f _(SP1) −f _(LO2)|  (6)

f _(SP2) ′=|f _(SP1) −f _(LO2) −Δf|  (7)

The frequency setting unit 30 decides the first intermediate frequency f_(IF1) of the first intermediate frequency signal S_(IF1) and the frequency f_(LO1) of the first local oscillation signal S_(LO1) in accordance with the center frequency f_(c) of the measured signal S_(RF) depending on Expressions (1) and (4) above. Furthermore, the frequency setting unit 30 sets the decided frequency f_(LO1) and the frequency f_(LO2), which is a fixed value, in the first local oscillator 11 and the second local oscillator 14, respectively.

Here, the frequency setting unit 30 decides the frequency shift amount Δf that does not establish a relationship in Expression (8) below, in accordance with the center frequency f_(c) of the measured signal S_(RF) such that the frequency f_(SP2)′ of the second spurious signal S_(SP2)′ after the frequency shift performed by the NCOM unit 26 is not included in the passed bandwidth W of the digital filter 27. In addition, the frequency setting unit 30 calculates Expressions (1) to (7) using the decided frequency shift amount Δf.

f _(IF2) ′−W/2≤f _(SP2) ′≤f _(IF2) ′+W/2−W/2≤|f _(SP1) −f _(LO2) −Δf|−f _(IF2) ′≤+W/2  (8)

For example, in a case where a relationship in Expression (9) below is not established, the frequency setting unit 30 decides the frequency shift amount Δf as Δf=0. Meanwhile, in a case where the relationship in Expression (9) below is established, the frequency setting unit 30 decides the frequency shift amount Δf that satisfies Δf≥2W/3 or Δf≤−2W/3.

f _(IF2) ′−W/2≤f _(SP2) ≤f _(IF2) ′+W/2−W/2≤|f _(SP1) −f _(LO2) |−f _(IF2) ′≤+W/2  (9)

For example, an example of f_(c), f_(SP1), Δf, f_(IF1), f_(LO1), f_(LO2), f_(SP2), f_(IF2), f_(SP2)′, f_(IF2)′, and a value of a passed band of the digital filter 27 in a case where the passed center frequency f₀ of the BPF 13 is 9 GHz and the passed center frequency f_(D) of the digital filter 27 is 1.95 GHz is illustrated in FIG. 2. Here, in a case where the passed bandwidth W of the digital filter 27 is 190 MHz, the passed band of the digital filter 27 is 1.855 to 2.045 GHz centered at f_(IF2)′=1.95 GHz.

In this example, in a case where the center frequency f_(c) of the measured signal S_(RF) is within a range of 4.45 GHz≤f_(c)≤4.5 GHz, setting 0.2 GHz as the frequency shift amount Δf not establishing the relationship in Expression (8) results in the first intermediate frequency f_(IF1) of 9.2 GHz. Furthermore, the NCOM unit 26 performs the frequency shift of the second intermediate frequency signal S_(IF2) and the second spurious signal S_(SP2) by 0.2 GHz.

In addition, in a case where the center frequency f_(c) of the measured signal S_(RF) is within a range of 4.5 GHz<f_(c)≤4.55 GHz, setting −0.2 GHz as the frequency shift amount Δf not establishing the relationship in Expression (8) results in the first intermediate frequency f_(IF1) of 8.8 GHz. Furthermore, the NCOM unit 26 performs the frequency shift of the second intermediate frequency signal S_(IF2) and the second spurious signal S_(SP2) by −0.2 GHz.

Consequently, the frequency f_(SP2)′ of the second spurious signal S_(SP2)′ after the frequency shift performed by the NCOM unit 26 deviates out of a range of f_(IF2)′−W/2≤f_(SP2)′≤f_(IF2)′+W/2, that is, out of a range of 1.855 to 2.045 GHz. Meanwhile, in a case where the center frequency f_(c) of the measured signal S_(RF) is within a range of f_(c)<4.45 GHz or 4.55 GHz<f_(c), the relationship in Expression (9) is not established. Thus, Δf=0 is satisfied.

For example, in a case where the center frequency f_(c) of the measured signal S_(RF) is 3 GHz, the frequency f_(SP2) of the second spurious signal S_(SP2) is not within a range of 1.855 to 2.045 GHz. Thus, the frequency shift amount Δf is set to Δf=0 Hz. Here, the frequency f_(LO1) is 12 GHz, and the first intermediate frequency f_(IF1) is 9 GHz. The first spurious signal S_(SP1) having the frequency f_(SP1) of 6 GHz is converted into the second spurious signal S_(SP2) having the frequency f_(SP2) of 4.95 GHz by the second mixer 15. In addition, the first intermediate frequency signal S_(IF1) having the first intermediate frequency f_(IF1) of 9 GHz is converted into the second intermediate frequency signal S_(IF2) having the second intermediate frequency f_(IF2) of 1.95 GHz by the second mixer 15.

Thus, the second spurious signal S_(SP2) after the AD conversion performed by the AD converter 20 passes through the NCOM unit 26 and then, is removed by the digital filter 27. Meanwhile, the second intermediate frequency signal S_(IF2) after the AD conversion performed by the AD converter 20 passes through the NCOM unit 26 and then, passes through the digital filter 27.

In addition, in a case where the center frequency f_(c) of the measured signal S_(RF) is 4.5 GHz, the frequency f_(SP2) of the second spurious signal S_(SP2) is within a range of 1.855 to 2.045 GHz. Thus, the frequency shift amount Δf is set to, for example, Δf=0.2 GHz. Here, the frequency f_(LO1) is 13.7 GHz, and the first intermediate frequency f_(IF1) is 9.2 GHz. The first spurious signal S_(SP1) having the frequency f_(SP1) of 9 GHz is converted into the second spurious signal S_(SP2) having the frequency f_(SP2) of 1.95 GHz by the second mixer 15. In addition, the first intermediate frequency signal S_(IF1) having the first intermediate frequency f_(IF1) of 9.2 GHz is converted into the second intermediate frequency signal S_(IF2) having the second intermediate frequency f_(IF2) of 1.75 GHz by the second mixer 15.

The second spurious signal S_(SP2) after the AD conversion performed by the AD converter 20 is converted into the second spurious signal S_(SP2)′ having the frequency f_(SP2)′ of 2.15 GHz by the NCOM unit 26 and then, is removed by the digital filter 27. Meanwhile, the second intermediate frequency signal S_(IF2) after the AD conversion performed by the AD converter 20 is converted into the second intermediate frequency signal S_(IF2)′ having the frequency f_(IF2)′ of 1.95 GHz by the NCOM unit 26 and then, passes through the digital filter 27.

FIGS. 3A and 3B are diagrams schematically illustrating a frequency relationship among a band of the measured signal S_(RF) immediately after being output from the BPF 13, a band of the first spurious signal S_(SP1), and the passed bandwidth W of the digital filter 27 in the example illustrated in FIG. 2. In this example, a bandwidth of the measured signal S_(RF) is 100 MHz. Thus, a bandwidth of the first spurious signal S_(SP1) is 200 MHz. In addition, a measurement bandwidth of the measured signal S_(RF) in the signal analysis unit 40 of a rear stage and the passed bandwidth W of the digital filter 27 are 190 MHz.

FIG. 3A illustrates the frequency relationship in a case where the center frequency f_(c) of the measured signal S_(RF) is within a range of 4.45 GHz≤f_(c)≤4.5 GHz and Δf is set to 0.19 GHz. In this case, the band of the first spurious signal S_(SP1) does not overlap with a measurement band of the measured signal S_(RF) at all times. In addition, as the center frequency f_(c) of the measured signal S_(RF) is decreased below 4.5 GHz, the band of the first spurious signal S_(SP1) moves away from the measurement band of the measured signal S_(RF).

FIG. 3B illustrates the frequency relationship in a case where the center frequency f_(c) of the measured signal S_(RF) is within a range of 4.5 GHz<f_(c)≤4.55 GHz and Δf is set to −0.19 GHz. Even in this case, the band of the first spurious signal S_(SP1)does not overlap with the measurement band of the measured signal S_(RF) at all times. In addition, as the center frequency f_(c) of the measured signal S_(RF) is increased above 4.5 GHz, the band of the first spurious signal S_(SP1) moves away from the measurement band of the measured signal S_(RF).

The signal analysis unit 40 illustrated in FIG. 1 analyzes the second intermediate frequency signal S_(IF2)′ passing through the digital filter 27 and includes a storage unit 41 and a baseband module 42.

The storage unit 41 stores digital data of the second intermediate frequency signal S_(IF2)′ passing through the digital filter 27.

The baseband module 42 generates orthogonal signals I(t) and Q(t) that are orthogonal to each other, by orthogonally demodulating the digital data of the second intermediate frequency signal S_(IF2)′ stored in the storage unit 41. In addition, the baseband module 42 performs a predetermined signal analysis process on the generated orthogonal signals I(t) and Q(t). Examples of the signal analysis process executed by the baseband module 42 include temporally changing an amplitude (power), a phase, a frequency, or the like of the measured signal SRF and obtaining a channel power (CHP), an occupied bandwidth (OBW), an adjacent channel leakage ratio (ACLR), a burst average power, modulation accuracy (EVM), a spectrum emission mask (SEM), a transmission power level, and a transmission spectrum mask.

The display unit 50 is configured with a display device such as an LCD or a CRT and displays various display contents such as a result of the signal analysis process performed by the signal analysis unit 40 in accordance with a display control performed by the control unit 52. Furthermore, the display unit 50 displays operation targets such as a button, a soft key, a pull-down menu, and a text box for setting a measurement condition and the like in accordance with a control signal output from the control unit 52.

The operation unit 51 receives an operation input of a user and is configured with, for example, a touch panel that is disposed on a surface of a display screen of the display unit 50. Alternatively, the operation unit 51 may be configured to include an input device such as a keyboard or a mouse. The operation input provided to the operation unit 51 is detected by the control unit 52. For example, the user can arbitrarily designate the center frequency f_(c) of the measured signal S_(RF), the passed center frequency f₀ of the BPF 13, and the passed bandwidth W of the digital filter 27 using the operation unit 51.

For example, the control unit 52 is configured with a microcomputer or a personal computer including a CPU, a ROM, a RAM, an HDD, and the like and controls an operation of each of the units constituting the signal analysis apparatus 1. In addition, at least a part of the frequency setting unit 30 and the baseband module 42 can be configured in a software manner by causing the control unit 52 to move a predetermined program stored in the ROM or the like to the RAM and execute the predetermined program. At least a part of the frequency setting unit 30 and the baseband module 42 can be configured with a digital circuit such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). Alternatively, at least a part of the frequency setting unit 30 and the baseband module 42 can be configured by appropriately combining a hardware process performed by the digital circuit and a software process performed by the predetermined program.

Hereinafter, an example of a process of the signal analysis method using the signal analysis apparatus 1 of the present embodiment will be described with reference to the flowchart in FIG. 4.

First, various information related to the frequency conversion, that is, information such as the center frequency f_(c) of the measured signal S_(RF), the passed center frequency f₀ of the BPF 13, and the passed bandwidth W of the digital filter 27, are input by the operation input provided to the operation unit 51 by the user (step S1).

Next, the frequency setting unit 30 decides the frequency shift amount Δf not establishing the relationship in Expression (8) above in accordance with the center frequency f_(c) of the measured signal S_(RF). Furthermore, the frequency setting unit 30 decides the first intermediate frequency f_(IF1) of the first intermediate frequency signal S_(IF1) and the frequency _(fLol) of the first local oscillation signal S_(LO1) in accordance with the center frequency f_(c) of the measured signal S_(RF) depending on Expressions (1) and (4) above (frequency setting step S2).

Next, the first local oscillator 11 outputs the first local oscillation signal S_(LO1) of the frequency f_(LO1) (first local oscillation step S3).

Next, the first mixer 12 generates the first intermediate frequency signal S_(IF1) of the first intermediate frequency f_(IF1) by performing the frequency mixing of the measured signal S_(RF) with the first local oscillation signal S_(LO1), and generates the first spurious signal S_(SP1) having the frequency f_(SP1) of the second harmonic component of the measured signal S_(RF) (first frequency mixing step S4).

Next, the output signals including the first intermediate frequency signal S_(IF1) and the first spurious signal S_(SP1) output from the first mixer 12 are input into the BPF 13, and the BPF 13 allows the first intermediate frequency signal S_(IF1) and the first spurious signal S_(SP1) to pass (bandpass step S5).

Next, the second local oscillator 14 outputs the second local oscillation signal S_(LO2) of the frequency f_(LO2) (second local oscillation step S6).

Next, the second mixer 15 generates the second intermediate frequency signal S_(IF2) by performing the frequency mixing of the first intermediate frequency signal S_(IF1) passing through bandpass step S5 with the second local oscillation signal S_(LO2). In addition, the second mixer 15 generates the second spurious signal S_(SP2) by performing the frequency mixing of the first spurious signal S_(SP1) passing through bandpass step S5 with the second local oscillation signal S_(LO2) (second frequency mixing step S7).

Next, the AD converter 20 performs the AD conversion of the second intermediate frequency signal SIF2 and the second spurious signal SSP2 output from the second mixer 15 (AD conversion step S8).

Next, the NCOM unit 26 performs the frequency shift of the second intermediate frequency signal S_(IF2) and the second spurious signal S_(SP2) after the AD conversion in AD conversion step S8 by the frequency shift amount Δf on the frequency axis (NCOM step S9). Here, the second intermediate frequency f_(IF2)′ of the second intermediate frequency signal S_(IF2)′ after the frequency shift performed in NCOM step S9 is provided as |f₀−f_(LO2)| as illustrated in Expression (3). In addition, the second spurious signal S_(SP2)′ after the frequency shift performed in NCOM step S9 is provided as |f_(SP1)−f_(LO2)−Δf| as illustrated in Expression (7). In addition, NCOM step S9 may be referred to as frequency shift unit.

Next, the digital filter 27 allows the second intermediate frequency signal S_(IF2)′ after the frequency shift in NCOM step S9 to pass and removes the second spurious signal S_(SP2)′ after the frequency shift in NCOM step S9 (digital filter step S10).

Next, the signal analysis unit 40 analyzes the second intermediate frequency signal S_(IF2)′ passing through digital filter step S10 (signal analysis step S11).

As described above, the signal analysis apparatus 1 according to the present embodiment decides the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1) in accordance with the center frequency fc of the measured signal S_(RF) depending on Expressions (1), (4), (8), and the like, performs an analog signal process using the first frequency conversion unit 10, and then, performs a digital signal process using the second frequency conversion unit 25. Accordingly, by varying the frequency shift amount Δf, the first intermediate frequency f_(IF1), and the frequency f_(LO1), the signal analysis apparatus 1 according to the present embodiment can analyze the measured signal S_(RF) while avoiding a spurious harmonic of the measured signal S_(RF) generated by the first mixer 12, without changing a configuration of hardware related to the frequency conversion. Furthermore, even in a case where a standard that uses a new band is established in the future, the signal analysis apparatus 1 according to the present embodiment can perform measurement that avoids a spurious harmonic, without changing the configuration of the hardware related to the frequency conversion.

In addition, since the first intermediate frequency f_(IF1) and the frequency f_(LO1) can be separately used by shifting the first intermediate frequency f_(IF1) and the frequency f_(LO1) by the frequency shift amount Δf, the signal analysis apparatus 1 according to the present embodiment can receive a desired wave of the second intermediate frequency f_(IF2)′ while removing an unnecessary spurious harmonic of the frequency f_(SP2)′ using the digital filter 27 of the passed bandwidth W. In other words, the signal analysis apparatus 1 according to the present embodiment can separate the first intermediate frequency f_(IF1) and the frequency f_(SP1) of the first spurious signal S_(SP1) generated by the first mixer 12, using the digital filter 27 of the rear stage.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1: signal analysis apparatus

10: first frequency conversion unit

11: first local oscillator

12: first mixer

13: BPF

14: second local oscillator

15: second mixer

20: AD converter

25: second frequency conversion unit

26: NCOM unit

27: digital filter

30: frequency setting unit

40: signal analysis unit

100: DUT 

What is claimed is:
 1. A signal analysis apparatus that analyzes a measured signal (S_(RF)) having a frequency component f_(RF), the signal analysis apparatus comprising: a frequency setting unit that decides a first intermediate frequency f_(IF1), a frequency f_(LO1) of a first local oscillation signal, and a frequency shift amount Δf in accordance with a center frequency f_(c) of the measured signal; a first local oscillator that outputs the first local oscillation signal (S_(LO1)); a first mixer that generates a first intermediate frequency signal (S_(IF1)) of the first intermediate frequency f_(IF1) by performing frequency mixing of the measured signal with the first local oscillation signal and generates a first spurious signal (S_(SP1)) including a second harmonic component of the measured signal; a bandpass filter of a passed center frequency f₀ into which the first intermediate frequency signal and the first spurious signal are input; a second local oscillator that outputs a second local oscillation signal (S_(LO2)) of a frequency f_(LO2); a second mixer that generates a second intermediate frequency signal (S_(IF2)) of a second intermediate frequency f_(IF2) by performing frequency mixing of the first intermediate frequency signal passing through the bandpass filter with the second local oscillation signal and generates a second spurious signal (S_(SP2)) by performing frequency mixing of the first spurious signal passing through the bandpass filter with the second local oscillation signal; an AD converter that performs AD conversion of the second intermediate frequency signal and the second spurious signal; a frequency shift unit that performs frequency shift of the second intermediate frequency signal and the second spurious signal after the AD conversion performed by the AD converter by the frequency shift amount Δf on a frequency axis; a digital filter of a passed bandwidth W that allows a second intermediate frequency signal (S_(IF2)′) after the frequency shift performed by the frequency shift unit to pass and removes a second spurious signal (S_(SP2)′) after the frequency shift performed by the frequency shift unit; and a signal analysis unit that analyzes the second intermediate frequency signal passing through the digital filter, wherein the first intermediate frequency f_(IF1) is provided as f₀+Δf, the second intermediate frequency f_(IF2) is provided as |f₀+Δf−f_(LO2)|, a second intermediate frequency f_(IF2)′ of the second intermediate frequency signal after the frequency shift performed by the frequency shift unit is provided as |f₀−f_(LO2)|, the frequency f_(LO1) is provided as f_(c)+f₀+Δf, a frequency f_(SP1) of the first spurious signal is provided as 2×f_(RF), a frequency f_(SP2) of the second spurious signal is provided as |f_(SP1)−f_(LO2)|, a frequency f_(SP2)′ of the second spurious signal after the frequency shift performed by the frequency shift unit is provided as |f_(SP1)−f_(LO2)−Δf|, and the frequency setting unit decides the frequency shift amount Δf that does not establish a relationship of −W/2≤|f_(SP1)−f_(LO2)−Δf|−f_(IF2)′≤+W/2, and decides the first intermediate frequency f_(IF1) and the frequency f_(LO1).
 2. The signal analysis apparatus according to claim 1, wherein in a case where a relationship of −W/2≤|f_(SP1)−f_(LO2)|−f_(IF2)′≤+W/2 is not established, the frequency setting unit decides the frequency shift amount Δf as Δf=0.
 3. The signal analysis apparatus according to claim 1, wherein in a case where a relationship of −W/2≤|f_(SP1)−f_(LO2)|≤f_(IF2)′≤+W/2 is established, the frequency setting unit decides the frequency shift amount Δf capable of allowing the second intermediate frequency signal to pass and removing the second spurious signal at the digital filter.
 4. A signal analysis method for analyzing a measured signal (S_(RF)) of a frequency f_(RF), the signal analysis method comprising: a frequency setting step (S2) of deciding a first intermediate frequency f_(IF1), a frequency f_(LO1) of a first local oscillation signal, and a frequency shift amount Δf in accordance with a center frequency f_(c) of the frequency f_(RF); a first local oscillation step (S3) of outputting the first local oscillation signal (S_(LO1)) of the frequency f_(LO1); a first frequency mixing step (S4) of generating a first intermediate frequency signal (S_(IF1)) of the first intermediate frequency f_(IF1) by performing frequency mixing of the measured signal with the first local oscillation signal, and generating a first spurious signal (S_(SP1)) including a second harmonic component of the measured signal; a bandpass step (S5) of a passed center frequency f₀ into which the first intermediate frequency signal and the first spurious signal are input; a second local oscillation step (S6) of outputting a second local oscillation signal (S_(LO2)) of a frequency f_(LO2); a second frequency mixing step (S7) of generating a second intermediate frequency signal (S_(IF2)) of a second intermediate frequency f_(IF2) by performing frequency mixing of the first intermediate frequency signal passing through the bandpass step with the second local oscillation signal, and generating a second spurious signal (S_(SP2)) by performing frequency mixing of the first spurious signal passing through the bandpass step with the second local oscillation signal; an AD conversion step (S8) of performing AD conversion of the second intermediate frequency signal and the second spurious signal; a frequency shift step (S9) of performing frequency shift of the second intermediate frequency signal and the second spurious signal after the AD conversion performed in the AD conversion step by the frequency shift amount Δf on a frequency axis; a digital filter step (S10) of a passed bandwidth W in which a second intermediate frequency signal (S_(IF2)′) after the frequency shift performed in the frequency shift step is allowed to pass, and a second spurious signal (S_(SP2)′) after the frequency shift performed in the frequency shift step is removed; and a signal analysis step (S11) of analyzing the second intermediate frequency signal passing through the digital filter step, wherein the first intermediate frequency f_(IF1) is provided as f₀+Δf, the second intermediate frequency f_(IF2) is provided as |f₀+Δf−f_(LO2)|, a second intermediate frequency f_(IF2)′ of the second intermediate frequency signal after the frequency shift performed in the frequency shift step is provided as |f₀−f_(LO2)|, the frequency f_(LO1) is provided as f_(c)+f₀+Δf, a frequency f_(SP1) of the first spurious signal is provided as 2×f_(RF), a frequency f_(SP2) of the second spurious signal is provided as |f_(SP1)−f_(LO2)|, a frequency f_(SP2)′ of the second spurious signal after the frequency shift performed in the frequency shift step is provided as |f_(SP1)−f_(LO2)−Δf|, and in the frequency setting step, the first intermediate frequency f_(IF1) and the frequency f_(LO1) are decided using the frequency shift amount Δf that does not establish a relationship of −W/2≤|f_(SP1)−f_(LO2)−Δf|−f_(IF2)′≤+W/2. 